Kinetics of precipitation in power plant steels

The ability of steels to resist creep deformation depends on the presence in the microstructure of carbide and intermetallic compounds which precipitate during tempering or during elevated temperature service. The precipitation occurs in a sequence which leads towards thermodynamic equilibrium. Kinetic theory has recently been developed which enables such sequences to be modelled using an adaptation of the classical Johnson-Mehl-Avrami equations. Our modification permits the treatment of more than one precipitation reaction occurring simultaneously, a feature which is essential for the reactions observed experimentally in a wide range of secondary hardening steels. In this paper, the model is applied to a series of power plant steels. It is possible to explain some interesting differences between the steels as a function of the chemical composition.